1. Field of the Invention
The present invention relates to an apparatus and method for acquiring initial synchronization at a high data rate in a satellite communication/broadcasting system in which a low Signal to Noise Ratio (SNR) and a high frequency difference exist.
2. Description of the Related Art
In order to accomplish proper communication in communication systems, especially in Time Division Multiplexing (TDM)/Time Division Multiple Access (TDMA) digital transmission systems, estimation and compensation of a carrier frequency error occurring due to frame synchronization technology of determining a beginning point of a frame in a receiver end and imperfection of a local oscillator of a transmission and reception end are definitely required. In general, the performance of a frame and frequency synchronization process performed right after symbol timing synchronization in an initial synchronization mode is an important parameter acting as the basic assumption for the performance required for the entire system.
Recently, DVB-S2 (Digital Video Broadcasting-Satellite, Second Generation) and DVB-RCS (Digital Video Broadcasting-Return Channel System), which are representative digital transmission standards using a TDM/TDMA scheme, satisfy a low bit error rate even in a poor channel environment, such as a low SNR, by introducing a robust channel code, such as a Low-Density Parity-Check (LDPC) code or a turbo code, enabling communication with a high data rate and high quality. For example, in a case of DVB-S2 system, which is a next generation satellite communication/broadcasting system, when Quadrature Phase Shift Keying (QPSK) modulation method is used, a bit frame is 64,800, and a code rate is ¼, a Packet Error Rate (PER) is about 10−7 at ES/No=−2.35 dB, i.e., a high quality communication system can be achieved. In addition, Since a carrier frequency usually corresponds to Ku/Ka band when satellite communication and a high frequency are used, a frequency error based on a bandwidth is relatively very high in comparison to other systems. Furthermore, a Doppler frequency error occurring in a high-speed mobile environment, such as high-speed train or airplane, is also largely generated. Actually, in the case of DVB-S2 system, a considered frequency error is ±5 MHz, and thus, when a bandwidth is considered as 25 MHz, a frequency error of ±20% based on the bandwidth is generated.
Moreover, in a case of DVB-RCS system, a symbol rate may be hundreds Ksps in general due to a variety of return link traffic. In this case, if a Doppler frequency error of 1,000 Km/h is generated in an airplane, the maximum frequency error of ±10% based on a bandwidth is generated. In particular, in a case of CSC/ACQ/SYNC burst for initial connection and synchronization, it is expected that a frequency error of more than several percentages is always generated. Due to these channel performance degradation causes, it is difficult to establish initial synchronization at a high data rate, and thus, the present invention suggests a method of simultaneously performing frame synchronization and frequency synchronization in order to overcome this difficulty.
In recent wireless communication systems including the DVB-S2/DVB-RCS systems, the development of robust channel code technology, such as LDPC, allows the systems to enable a broadband (high data rate) and high quality service. However, a conventional initial synchronization scheme causes problems that initial synchronization is significantly delayed due to a low SNR and a high frequency error in a channel and a demodulation circuit of a receiver end for the initial synchronization is complicated.